Reductive carbonylation of nitroarenes and/or arylamines in the presence of various catalysts is an effective strategy for preparing N,N'-diarylureas. For example, Heck et al. (J. Org. Chem. 40 (1975) 2819) describe the synthesis of various symmetric diarylureas from the reaction of nitroarenes, aromatic amines, and carbon monoxide in the presence of a palladium catalyst. The authors also note, however, that complicated mixtures of ureas result when aromatic amines and nitroarenes containing different substituents react in this process. For example, the reaction of aniline with p-chloronitrobenzene gives N,N'-diphenylurea and N,N'-bis(p-chlorophenyl)urea in addition to the expected Nphenyl-N'-(p-chlorophenyl)urea.
Similarly, Watanabe et al. (J. Organometal. Chem. 290 (1985) 249) reported that n-butylamine reacts with nitrobenzene and carbon monoxide in the presence of a platinum compound to give a mixture of N,N'-dibutylurea (46%), N-butyl-N'-phenylurea (46%), and N,N'-diphenylurea (6%).
UK Patent Application GB 2 233 973A teaches a process in which a primary aliphatic amine reacts with a nitroarene and carbon monoxide in the presence of a rhodium catalyst to give only the symmetric N,N'-dialkylurea and a primary aromatic amine. No unsymmetric urea is produced. When a secondary aliphatic amine is used in place of the primary aliphatic amine, only traces of urea products are detected.
East German Patent No. 227,700 (Chem. Abstracts 104 224726v) teaches to prepare N,N-dimethyl-N'-phenylurea from nitrobenzene, CO, and dimethylamine in the presence of a sulfur/vanadium pentoxide catalyst system. The reference teaches the criticality of the vanadium pentoxide, and does not suggest using a Group VIII metal catalyst for the process.
The references discussed in the preceding paragraphs make it clear that a synthesis of an unsymmetric urea from a nitroarene and a secondary amine using a Group VIII transition metal catalyst is likely to give, if anything, a complicated mixture of urea products. On the other hand, a selective process for producing trisubstituted ureas, especially N'-aryl-N,N-dialkylureas, is desirable because these ureas are easily cracked in the presence of various promoters to give aryl isocyanates.